1. Field of the Invention
This invention relates generally to a photonic crystal gain amplifier for a telecommunications system and, more particularly, to a photonic crystal gain amplifier for an optical telecommunications system, where the amplifier employs a photonic crystal gain medium that only allows a fundamental mode to propagate therethrough at both the pump and signal wavelengths, and where the mode field diameter at both wavelengths is much wider than the corresponding mode fields present in a fundamental-mode fiber.
2. Discussion of the Related Art
Optical communications systems employ optical transmission fibers to transmit optical signals carrying information over great distances. An optical fiber is an optical waveguide including a core having one index of refraction surrounded by a cladding having another, lower, index of refraction so that light signals propagating down the core at a certain angle of incidence are trapped therein. Typical optical fibers are made of high purity silica including certain dopant atoms that control the index of refraction of the core and cladding.
The optical signals are separated into optical packets to distinguish groups of information. Different techniques are known in the art to identify the optical packets transmitted through an optical fiber. These techniques include time-division multiplexing (TDM) and wavelength-division multiplexing (WDM). In TDM, different slots of time are allocated for the various packets of information. In WDM, different wavelengths of light are allocated for different data channels carrying the optical packets. More particularly, sub-bands within a certain bandwidth of light are separated by predetermined wavelengths to identify the various data channels.
When optical signals are transmitted over great distances through optical fibers, attenuation within the fibers reduces the optical signal strength. Therefore, detection of the optical signals over background noise becomes more difficult at the receiver. In order to overcome this problem, optical fiber amplifiers are positioned at predetermined intervals along the fiber, for example, every 80-100 km, to provide optical signal gain. Various types of fiber amplifiers are known that provide an amplified replica of the optical signal, and provide amplification for the various modulation schemes and bit-rates that are used.
A popular optical fiber amplifier for this purpose is an erbium doped fiber amplifier (EDFA) that provides optical amplification over the desired transmission wavelengths. EDFAs are common because erbium atoms provide light amplification over a relatively broad wavelength range, for example, 1525-1610 nm. The erbium-doped fiber within the EDFA is pumped by a pump laser at a certain excitation frequency, such as 980 nm or 1480 nm. These wavelengths are within the absorption band of the erbium, and results in the generation of optical gain in the wavelength range of 1550 nm. Thus, for an optical signal with a center wavelength at about 1550 nm propagating through the erbium-doped fiber, the signal is amplified by the stimulated emission of 1550 nm energy when the fiber is pumped by a 980 nm pump source. The pump light is absorbed by the erbium atoms that cause electrons in the atoms to be elevated to higher states. When a photon in the optical signal being transmitted hits an excited erbium atom, a photon of the same wavelength and at the same phase is emitted from an elevated electron, which causes the electron to decay to a lower state to again be excited to a higher state by the pump photons. The optical signal is amplified by the generation of additional photons in this manner.
Another type of fiber amplifier sometimes employed in a fiber communications link is a Raman amplifier. A Raman amplifier provides amplification within the fiber itself by launching pump light into the fiber from a pump source. The pump light raises the energy state of electrons in the dopant atoms within the fiber that then emit light at the wavelength of the optical signal. Semiconductor lasers are generally used in the pump source to generate the pump light, and a wavelength division multiplexer (WDM) is used to couple the pump light into the fiber. Typically, the wavelength of the pump light is about 100 nm less than the wavelength of the signal light to provide the amplification. For example, to amplify signal light in the C and L bands (1520-1600 nm), lasers generating pump light in the 1420-1500 nm wavelengths are used.
The pump light can be launched in either the co-propagating or counter-propagating direction relative to the propagation direction of the optical signal. However, counter-propagating pump light typically has advantages over co-propagating pump light. Most optical communications systems employing Raman amplification take advantage of the counter-propagating pump configuration, where the pump light propagates in the opposite direction to the signal light. Counter-propagating the pump light has the advantage of vastly reducing the amount of pump noise transferred onto the signal channels, as well as minimizing the problem of pump-mediated cross-talk. As reach and information capacity of transmission systems are pushed into even higher limits, the desire to utilize both co-propagating and counter-propagating Raman pump configurations is increasing. Co-propagating Raman pumping gives system performance benefits because the signal powers are maintained at a more uniform power level to route each span of the system.
Another type of optical fiber amplifier employs photonic crystals as the gain medium in the amplifier. Two dimensional photonic crystals are materials containing periodically varying indices of refraction which limit the number of optical modes that are allowed to propagate in much the same way that a fiber has a limited number of allowable modes. Typically, two-dimensional photonic crystals are constructed with periodically spaced air gaps (holes) or layers of different materials or lattice sites that provide a periodic array of refractive index variation. For example, photonic crystal fibers (PCF) are constructed with periodic air holes in a cross-section which runs the length of the fiber, creating a photonic crystal with axial symmetry. A light signal will propagate in a fundamental mode in the central region of the fiber guided by the array of periodic holes or lattice sites formed adjacent to or surrounding the central core region. PCFs have been built with core diameters that are greater than 10 times that of a normal fundamental mode fiber, where all wavelengths above approximately 500 nm propagate as a fundamental mode.
A discussion of photonic crystal fibers can be found in T. A. Birks et al., xe2x80x9c2-D Photonic Bandgaps in Silica/Air Structures,xe2x80x9d Electronic Letters Vol. 31 (22) pp. 1941-1943, Oct. 26, 1995; J. C. Night et al., xe2x80x9cPure Silica Single Mode Fiber with Hexagonal Photonic Crystal Claddingxe2x80x9d Proceedings of OFC, pp. pd 3-1-pd 3-5, February, 1996; J. D. Joannopoulous et al., xe2x80x9cPhotonic Crystals: Moulding the Flow of Light,xe2x80x9d Chapter 5, Princeton University Press, 1995; and U.S. Pat. Nos. 5,784,440; 5,802,236; 6,097,870 and 6,175,671.
Bulk optical amplifiers employing a photonic crystal are typically separated into two types. A first type includes those amplifiers that collimate or otherwise focus the pump beam before entering the gain medium, and a second type that uses the wave guiding properties of a narrow gain region to avoid the need for pump collimation. Those designs that employ the first type require very precise optical alignment and complex beam-circularization optics, since pump laser sources provide outputs that are highly divergent, elliptical beam profiles, which is counter to the design of a low cost amplifier. Those designs that employ the second type do not require complicated optics for pump collimation, but suffer from poor performance because the pump and signal mode fields have poor overlaps, thus much of the pump beam is wasted in the regions where there is no signal beam, and the size of the gain medium is limited by the prohibitive pump power required to achieve the necessary power densities as the waveguide cross-section increases.
One of the unique properties of axial photonic crystals is that they propagate a fundamental or single mode of a much larger lateral size and broader wavelength range than is achievable in a conventional fundamental mode optical fiber. International Publication No. WO 99/00685, titled xe2x80x9cSingle Mode Optical Fibre,xe2x80x9d published Jan. 7, 1999, discloses a PCF where the core diameters are in the range of 50 Tm to 75 Tm where the light propagating in the core region is maintained in the fundamental mode. The standard fiber has a core diameter of 6 Tm. Such large core PCFs may be employed as bulk amplifiers for signal amplification in optical telecommunications systems for transmission of data signals. Other related articles include J. C. Night et al., xe2x80x9cLarge Mode Area Photonic Crystal Fiber,xe2x80x9d Electronic Letters, Vol. 34 (13), pp. 1347-1348, Jun. 25, 1998 and W. J. Wadsworth et al., xe2x80x9cYB3+-Doped Photonic Crystal Fiber Laser,xe2x80x9d Electronic Letters, Vol. 36 (17), pp. 1452-1454, Aug. 17, 2000.
Because PCFs have such a large core region as compared to conventional single mode fibers, the term xe2x80x9cphotonic crystal fiberxe2x80x9d may be somewhat of a misnomer in that these crystals function more in a bulk with a periodic waveguiding mechanism than as a fiber with a continuous waveguide mechanism in the form of a cladding. Herein, photonic crystals are referred to as a photonic crystal medium or, with the addition of a rare earth dopant in a core region, a photonic crystal gain medium.
In accordance with the teachings of the present invention, a photonic crystal amplifier is disclosed that employs a photonic crystal gain medium for a telecommunications system. Pump light is coupled into a core region of the amplifier at one end of the gain medium and signal light is coupled into and out of the amplifier at an opposite end of the gain medium. The pump light input end includes a wavelength reflector that reflects the signal wavelengths of light, but transmits the pump wavelengths of light. The signal light input/output end includes a pump wavelength reflector that reflects the pump wavelengths back into the gain medium, but is transparent to the signal wavelengths. Therefore, both the pump light and the signal light pass twice through the gain medium for enhanced gain.
The diameter of the core region of the gain medium is relatively large, but allows only a single mode to propagate at the pump and signal wavelengths. Because the photonic crystal medium allows both the pump and signal wavelengths to propagate as fundamental modes, they will have good overlap through the gain medium, providing greater amplification per length of the gain medium.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.